There are widely used, as a variety of food containers, molded products obtained by molding a sheet made of polyester resin, particularly polyethylene terephthalate. Of these, a polyester resin molded product increased in crystallizability to increase the heat resistance, is used as an ovenable tray. FIGS. 10a and 10b show a food container 60 as example of molded product.
Such a polyester resin molded product increased in crystallizability is obtained by once molding a sheet-like polyester resin with the use of a molding die of high temperature enabling the polyester resin to be crystallized, and by continuously heating the polyester resin to accelerate the crystallization. The reason why such processing to increase the crystallizability is conducted, is that if the polyester resin is merely molded, the polyester resin molded product has low crystallinity, which is not sufficiently heat-resistant. The following will discuss the reason why such a processing of increasing the crystallizability is conducted after the sheet has been molded. If the crystallinity of a sheet, before being molded, is high, the moldability of the sheet is lowered. Specifically, for example, such a higher crystallinity sheet is particularly poor in elongation at the time of molding. This disadvantageously prevents the sheet from being molded accurately with respect to details in the mold. Further, as compared with a foamed polyester resin molded product, the molded product thus obtained is relatively high in density and therefore uneconomical.
Each of U.S. Pat. No. 4,462,947 and U.S. Pat. No. 4,466,933 discuss a process for molding a foamed polyester resin sheet of which foaming magnification is relatively low, with the use of a molding die at high temperature enabling the polyester resin to be crystallized during molding, and continuously heating the sheet to accelerate the crystallization, thus obtaining food containers which are excellent in heat resistance. According to a general molding method, however, a foamed polyester resin sheet is heated to a crystallization temperature not only at that part thereof which results in a final product, but also in other portions thereof, which will later be trimmed. Accordingly, the sheet is considerably softened in its entirety and may readily be distorted even with a slight external force.
Further, as shown in FIG. 11, even after being molded with molding dies 13a, 13b, the sheet 30 which has been considerably softened in its entirety, generally hangs down, and is distorted due to its own weight. In addition, after the crystallization has been accelerated, it is difficult to readily release the foamed polyester resin sheet from the molding die at the high temperature above-mentioned. Accordingly, the resulting molded product is considerably distorted due to the external force exerted at the time of mold release or due to parts of the sheet hanging-down because of its own weight after having been molded. Such distortion is observed particularly in a process for molding, for example, a polyester resin sheet which has been highly foamed to a foaming magnification of 2 times or more. Continuously heating the sheet to accelerate the crystallization thereof, produces a molded product improved in heat resistance. The distortion becomes greater as the foaming magnification of the foamed polyester resin sheet is increased and as the temperature at which the crystallization is conducted is increased. More specifically, as the foaming magnification increases and as the temperature is made higher, the foamed sheet is more readily softened. It may be considered that, when a polyester resin sheet, foamed to a foaming magnification of 2 or more, is heated to such a high temperature as to enable the sheet to be crystallized, the sheet is remarkably distorted.
Accordingly, the molded sheet is not easy to release from a high temperature molding die without any distortion. Thus, if the sheet is forcibly released, the distortion thereof becomes great. Further, since the sheet is remarkably softened in its entirety even after having been molded, the sheet greatly hangs down and is distorted due to its own weight. To prevent such distortion, it is preferable to release the molded product, thus obtained with the use of one molding die, through the steps of molding a foamed polyester resin sheet under conditions sufficient to accelerate the crystallization thereof; and cooling the sheet to temperature (which is preferably not greater than the glass transition point) at which the sheet has sufficient strength to prevent the sheet from being distorted. For the following reasons, however, it is not suitable for industrial production to conduct the steps of molding, crystallizing and cooling in a single one molding die, i.e., to continuously conduct the steps of heating and cooling within one molding die. That is, such an arrangement requires much time which lengthens the molding cycle and increases the energy loss. In view of the foregoing, to achieve industrial production, it is required to separate the heating die from the cooling die.
When the foamed polyester resin sheet, once distorted after having been molded and accelerated in crystallization, is cooled, there is produced a molded product which is just as distorted. Accordingly, the sheet is often corrected in shape while the sheet is cooled in another cooling die. However, since the molded sheet is distorted and hangs down due to its own weight, it is very difficult to match the position of the molding portion of the cooling die with the position of the molded portion of the sheet. Mostly, these portions are positionally shifted from each other. Thus, there is only obtained a molded product which has been molded once again, and cooled as positionally shifted in a shape different from the shape at the time when the sheet was molded and crystallized. The re-molded product thus obtained is apparently good. However, the inventors have found the following. That is, the re-molded product memorizes the shape obtained at the time when the sheet was first molded and crystallized. Accordingly, when the re-molded product is heated for cooking with the use of an oven or the like, the re-molded product is returned to the originally memorized shape and therefore becomes distorted. Further, the inventors have found that the distortion is caused by the fact that the shape obtained at the time when the sheet was molded and crystallized, has been memorized by the molded product. Thus, the shape of the molded product when molded and crystallized, is memorized. Accordingly, to prevent the molded product from being distorted due to the application of heat thereto, it is important to very accurately maintain the shape of the molded product when molded and crystallized and when cooled. In particular, for a molded product such as an ovenable molded container having a lid to be fitted thereto even after the container has been heated for cooking, it is required to minimize such distortion in view of the lid fitting requirements.
In addition, such distortion of a molded product presents the following defects. It is not possible to assure the internal volume of the contents required for the target molded product. Further, the external appearance is different from that of the target molded product. The respective molded products are different in shape from one another, so that the commercial values thereof are lost. In particular, when a molded product is considerably distorted, the contents thereof may be spilt due to the application of heat when cooked. This contaminates the inside of the oven, presenting a functional defect for a heat-resistant food container.
As a result of study to overcome the defects above-mentioned, the inventors have accomplished the present invention based on the finding that, when a foamed polyester resin sheet of low crystallinity is molded and only that part of the sheet which results in a final product is heated and crystallized, and simultaneously the unheated and therefore uncrystallized portion of the sheet is cooled, a resulting molded product can readily be released from the molding die without it being distorted, and the remaining foamed polyester resin sheet even after released from the molding die, does not hang down due to its own weight.